Energy saving method in combined system of bioethanol producing device and solid oxide fuel cell

ABSTRACT

The present invention is to provide, in a combined system of a bioethanol producing device and an SOFC, a method that is capable of further enhancing the electric power generation efficiency of the SOFC, and is also capable of achieving further reduction of the energy required for distillation of the fermented liquid. A part of an anode off-gas is refluxed to the water-containing ethanol vapor line from the mash column to the reforming device at a reflux ratio ((flow rate of reflux gas)/(flow rate of (anode off-gas)−(reflux gas))) of from 1 to 2. The ethanol concentration of the water-containing ethanol vapor is controlled by refluxing, to a range of from 25 to 35% by weight with water contained in the anode off-gas of the solid oxide fuel cell.

TECHNICAL FIELD

The present invention relates to an energy saving method in a combinedsystem of a bioethanol producing device and a solid oxide fuel cell(which may be hereinafter referred to as “SOFC”).

BACKGROUND ART

Bioethanol has a problem of consuming a large amount of energy forethanol enrichment and distillation-purification since an automobilefuel requires high concentration ethanol of 99.5% by weight or more.

An SOFC system is equipped with a reforming device that vaporizes andreforms an ethanol aqueous solution of from 25 to 35% by weight togenerate hydrogen gas, and an SOFC that generates electric power withthe hydrogen gas, and thus does not require a distillation-purificationcolumn.

Under the circumstances, some proposals have been made for a system thatintends to reduce the consumption energy cost and the equipment cost bycombining a bioethanol producing device and an SOFC.

For example, PTL 1 proposes a combined system equipped with a means forwithdrawing water-containing ethanol vapor having a concentration offrom 30 to 70% by weight from an overhead of a distillation column of abioethanol producing device, and a reforming means for producing areformed gas from the overhead vapor, and also equipped with a SOFCoperated with the reformed gas as a fuel.

PTL 2 describes a fuel cell system having a reforming device forming areformed gas containing hydrogen from a hydrocarbon raw materialsupplied through reformation reaction including steam reformationreaction, a fuel cell performing electric power generation by supplyingthe reformed gas to an anode, and an anode off-gas introducing means forintroducing at least a part of an anode off-gas containing water formedin association with the electric power generation to an inlet port ofthe reforming device through an ejector.

PTL 3 describes a fuel cell electric power generation system performingelectric power generation in such a manner that an ethanol fuel having aconcentration of from 15.4 to 46% by weight obtained from an ethanolfermented liquid is vaporized to form a mixed gas of steam and ethanol,which is reformed by being supplied to a reformation reaction zone togenerate a reformed gas containing hydrogen, and the reformed gas issupplied to an SOFC.

NPL 1 reports a study on a method for providing a hydrogen rich gassuitable for supplying to a fuel cell, through steam reformation ofbioethanol, in which FIG. 1 shows an electric power generation systemhaving a step of providing a water-containing ethanol vapor of from 45to 55% by weight through distillation of an ethanol fermented liquid, astep of reforming the water-containing ethanol vapor to form a reformedgas containing hydrogen, and a step of performing electric powergeneration with a fuel cell using the reformed gas as a fuel, and alsoshows that the heat of the off-gas discharged from the fuel cell is sentto and utilized in the distillation step and the reformation step.

CITATION LIST Patent Literatures

-   PTL 1: JP-A-2007-20407-   PTL 2: JP-A-2007-128680-   PTL 3: JP-A-2011-187328

Non Patent Literature

-   NPL 1: Catalysis Today, 75 (2002), pp. 145-155

SUMMARY OF THE INVENTION Problem to be Solved by the Invention

The techniques shown above as the background art achieve reduction ofthe consumption energy in the production of bioethanol to a certainextent but are still not satisfactory.

Under the circumstances, an object of the invention is to provide, in acombined system of a bioethanol producing device and an SOFC, a methodthat is capable of further enhancing the electric power generationefficiency of the SOFC, and is also capable of achieving furtherreduction of the energy required for distillation of the fermentedliquid.

Means for Solving the Problem

The invention according to claim 1 relates to an energy saving method ina combined system of a bioethanol producing device and a solid oxidefuel cell,

in a combined system containing a mash column that distills a fermentedliquid formed in a fermenting vessel of a bioethanol producing device todistill off a water-containing ethanol vapor from an overhead thereof, areforming device that forms a reformed gas from the water-containingethanol vapor, and a solid oxide fuel cell that is operated with thereformed gas as a fuel,

the method including refluxing a part of an anode off-gas of the solidoxide fuel cell to a water-containing ethanol vapor line from the mashcolumn to the reforming device at a reflux ratio ((flow rate of refluxgas)/(flow rate of (anode off-gas)−(reflux gas))) of from 1 to 2, so asto control an ethanol concentration of the water-containing ethanolvapor to a range of from 25 to 35% by weight with water contained in theanode off-gas of the solid oxide fuel cell.

The anode off-gas contains water generated through the electric powergeneration of the SOFC, and thus the ethanol concentration of thewater-containing ethanol vapor, which is to be supplied to the reformingdevice, can be controlled to a range of from 25 to 35% by weight withwater contained in the anode off-gas, by refluxing a part of the anodeoff-gas to the water-containing ethanol vapor line from the mash columnto the reforming device at the prescribed reflux ratio.

The reflux ratio is preferably in a range of from 1.2 to 1.8. When thereflux ratio is less than 1, the electric power generation efficiency ofthe SOFC may not be enhanced. When the reflux ratio is exceeding 1, theelectric power generation efficiency of the SOFC and the total thermalefficiency may be increased, but when the reflux ratio exceeds 2, theefficiencies may be saturated but may not be further increased.Furthermore, the increase of the reflux ratio requires the increase ofthe loads of the blower and the ejector, and also in this point of view,the upper limit of the reflux ratio is 2.

The invention according to claim 2 relates to the energy saving methodin a combined system of a bioethanol producing device and a solid oxidefuel cell according to claim 1, wherein the ethanol concentration of thewater-containing ethanol vapor distilled off from the mash column iscontrolled to a range of from 35 to 60% by weight.

The invention according to claim 3 relates to the energy saving methodin a combined system of a bioethanol producing device and a solid oxidefuel cell according to claim 1, wherein the ethanol concentration of thewater-containing ethanol vapor distilled off from the mash column iscontrolled to a range of from 55 to 60% by weight.

The invention according to claim 4 relates to the energy saving methodin a combined system of a bioethanol producing device and a solid oxidefuel cell according to any one of claims 1 to 3, wherein the balance ofthe anode off-gas of the solid oxide fuel cell and a cathode off-gasthereof are supplied to a catalytic combustor for the reforming deviceand a catalytic combustor for a reboiler of the mash column, so as tocombust a combustible component in the anode off-gas with oxygen in thecathode off-gas, and heat generated in the catalytic combustor for thereforming device is used for heating the reforming device, whereas heatgenerated in the catalytic combustor for the reboiler is used forheating a bottom liquid of the mash column.

The anode off-gas contains combustible components, such as H₂ (3 to 10%by volume) and CO (0 to 10% by volume), and the cathode off-gas containsfrom 5 to 10% by volume of oxygen. Accordingly, the combustiblecomponents are combusted with the oxygen in catalytic combustors, andheat generated therein is effectively used for heating the reformingdevice and the bottom liquid of the mash column. The catalytic combustorfor the reforming device and the catalytic combustor for the reboilereach may be a known one.

Advantageous Effects of Invention

1. For vaporizing and reforming ethanol in the reforming device throughsufficient and durable exhibition of the capability of the reformationcatalyst to generate hydrogen gas, it is necessary to control theethanol concentration of the water-containing ethanol vapor to a rangeof from 25 to 35% by weight.

In the invention according to claim 1, apart of the anode off-gas fromthe SOFC is refluxed to the water-containing ethanol vapor line from themash column to the reforming device at a reflux ratio ((flow rate ofreflux gas)/(flow rate of (anode off-gas)−(reflux gas))) of from 1 to 2,so as to control the ethanol concentration of the water-containingethanol vapor, which is to be supplied to the reforming device, to arange of from 25 to 35% by weight with water contained in the anodeoff-gas.

2. In an SOFC cell stack, the utilization rate of the combustiblecomponent (such as H₂ and CO) in the anode supply gas is generallyapproximately 70%. This is because the cell may be damaged if theconcentration of the combustible component in the anode supply gas istoo small.

In the invention according to claim 1, apart of the anode off-gas isrefluxed to the water-containing ethanol vapor line from the mash columnto the reforming device at the prescribed reflux ratio as describedabove, by which the concentration of the combustible component in theanode supply gas can be increased, so as to enhance the utilization rateof the fuel to approximately 90%, and thus the electric power generationefficiency can be enhanced.

3. In the reforming device, it is necessary to control the ethanolconcentration of the water-containing ethanol vapor to a range of from25 to 35% by weight, but in the case where a water-containing ethanolvapor that has an ethanol concentration in a range of from 25 to 35% byweight is to be distilled off from the overhead of the mash column, alarge amount of energy may be consumed. In the invention according toclaim 1, the ethanol concentration of the water-containing ethanol vaporcan be controlled to a range of from 25 to 35% by weight by refluxing apart of the anode off-gas to the water-containing ethanol vapor linefrom the mash column to the reforming device at the prescribed refluxratio as described above, by which the ethanol concentration of thewater-containing ethanol vapor thus distilled off from the mash columnmay be from 35 to 60% by weight, and further from 55 to 60% by weight,and thereby the consumption energy of the reboiler of the mash columncan be suppressed to the requisite minimum level.

4. In the invention according to claim 4, the balance of the anodeoff-gas of the SOFC and the cathode off-gas thereof are supplied to thecatalytic combustor for the reforming device and the catalytic combustorfor the reboiler of the mash column, so as to combust the combustiblecomponent (such as H₂ and CO) in the anode off-gas with oxygen in thecathode off-gas, and thus heat can be generated in the catalyticcombustor for the reforming device and the catalytic combustor for thereboiler, and can be effectively utilized for heating the reformingdevice and the bottom liquid of the mash column.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flow diagram showing an embodiment of the invention.

FIG. 2 is a graph showing the effect of refluxing the anode off-gas onthe efficiencies.

FIG. 3 is a graph showing the relationship between the overhead vaporconcentration and the consumption energy in the mash column.

EMBODIMENTS OF INVENTION

The invention will be described below with reference to embodimentsshown in the drawings.

In FIG. 1, the combined system of a bioethanol producing device and anSOFC according to the invention is constituted by the combination of amash column that distills a fermented liquid formed in a fermentingvessel of a bioethanol producing device for distilling off awater-containing ethanol vapor from an overhead thereof, a reformingdevice that forms a reformed gas from the water-containing ethanolvapor, and a solid oxide fuel cell that is operated with the reformedgas as a fuel.

In the system, a fermented liquid having an ethanol concentration of 5%by weight is supplied to the overhead of the mash column with a pump (1)through a heat recovery device (2). The mash column, for example, has anumber of distillation plates of 40, and distills the fermented liquidat a temperature in a range of from 100 to 154° C. The bottom liquid iswithdrawn from the bottom of the mash column, and a part of the bottomliquid is heated with a catalytic combustor and a reboiler and thenreturned to the bottom of the mash column, whereas the balance of thebottom liquid is discharged as waste water having an ethanolconcentration of less than 0.1% by weight through the heat recoverydevice (2).

On the other hand, a water-containing ethanol vapor having an ethanolconcentration of from 35 to 60% by weight is distilled off from theoverhead of the mash column at a temperature of from 90 to 130° C. and apressure of from the atmospheric pressure to 430 kPaG. Thewater-containing ethanol vapor is sent to the reforming device through aheat recovery device (3), and in the reforming device, ethanol isvaporized and reformed to generate a reformed gas containing hydrogen.The reformed gas is sent to the anode of the SOFC unit operated atapproximately 700° C., and utilized for electric power generation. Airis supplied to the cathode of the SOFC unit with a blower (4) through aheat recovery device (5), and oxygen in the air is used for electricpower generation.

A part of the anode off-gas is refluxed to an ejector (7) provided inthe course of the water-containing ethanol vapor line (6) from theoverhead of the mash column to the reforming device at a reflux ratio((flow rate of reflux gas)/(flow rate of (anode off-gas)−(reflux gas)))of approximately 1.5. The ethanol concentration of the water-containingethanol vapor is controlled by refluxing, to a range of from 25 to 35%by weight with water contained in the anode off-gas of the SOFC.

A part of the anode off-gas is thus refluxed to the water-containingethanol vapor line from the mash column to the reforming device at theprescribed reflux ratio as described above, by which the concentrationof the combustible component (such as H₂ and CO) in the anode supply gascan be increased, so as to enhance the utilization rate of the fuel toapproximately 90%, and thus the electric power generation efficiency canbe enhanced.

Furthermore, apart of the anode off-gas is thus refluxed to thewater-containing ethanol vapor line from the mash column to thereforming device at the prescribed reflux ratio as described above, bywhich the ethanol concentration of the water-containing ethanol vapor,which is distilled off from the mash column at a temperature of from 90to 130° C. and a pressure of from the atmospheric pressure to 430 kPaG,can be controlled to a range of from 25 to 35% by weight (assuming thatthe reflux ratio is from 1 to 2). Thus, the consumption energy of thereboiler of the mash column can be suppressed to the requisite minimumlevel.

The balance of the anode off-gas of the SOFC and the cathode off-gasthereof are combined, and at least a part of the combined gas isdiverted to pass through the catalytic combustor for the reformingdevice, the reforming device and the heat recovery device (3), and thencombined with the balance of the combined gas that is not diverted. Inthe catalytic combustor for the reforming device, the combustiblecomponent (such as H₂ and CO) derived from the anode off-gas iscombusted with oxygen derived from the cathode off-gas, and the heatthus generated is used for heating the reforming device. The combinedgas is also sent to the catalytic combustor for the reboiler of the mashcolumn, in which also the combustible component (such as H₂ and CO)derived from the anode off-gas is combusted with oxygen derived from thecathode off-gas, and the heat thus generated is used for heating thebottom liquid of the mash column.

FIG. 3 shows the relationship between the overhead vapor concentrationof the mash column and the consumption energy. The graph in the figureshows the result obtained by calculating the change of the reboiler heatquantity that is required for dehydrating and enriching a fermentedliquid having an ethanol concentration of 5% by weight at an overheadpressure of 50 kPaG, depending on the change of the overhead vaporconcentration. In the reforming device, for vaporizing and reformingethanol through sufficient and durable exhibition of the capability ofthe reformation catalyst to generate hydrogen gas, it is necessary tocontrol the ethanol concentration of the water-containing ethanol vaporto a range of from 25 to 35% by weight, and in the mash column, theconsumption energy is minimized under the condition thatwater-containing ethanol is enriched to an ethanol concentration in arange of from 35 to 60% by weight. The consumption energy is increasedunder the condition that water-containing ethanol is enriched to a rangeexceeding 60% by weight. The reboiler heat quantity is calculated byusing a process simulator (ASPEN). It is assumed that a fermented liquidhaving an ethanol concentration of 5% by weight is distilled atapproximately 90° C. with a number of distillation plates of thedistillation column of 40. In the region where the overhead vaporconcentration of the distillation column is in a low concentrationrange, an operation referred to as simple distillation is performed, andwith the increase of the concentration, the latent heat of vaporizationof water is decreased to reduce the reboiler heat quantity. When theconcentration is the certain value (which is approximately 55% by weightfound by trial calculation), further enrichment cannot be performed bysimple distillation, and a refluxing operation is required, whichincreases the reboiler heat quantity. In a range of enrichment of theconcentration higher than the value, the consumption energy is simplyincreased.

INDUSTRIAL APPLICABILITY

In a combined system of a bioethanol producing device and an SOFC, theinvention can be effectively applied thereto for further enhancing theelectric power generation efficiency of the SOFC and for furtherreducing the energy required for distillation of a fermented liquid.

1. An energy saving method in a combined system of a bioethanolproducing device and a solid oxide fuel cell, in a combined systemcontaining a mash column that distills a fermented liquid formed in afermenting vessel of a bioethanol producing device to distill off awater-containing ethanol vapor from an overhead thereof, a reformingdevice that forms a reformed gas from the water-containing ethanolvapor, and a solid oxide fuel cell that is operated with the reformedgas as a fuel, the method comprising refluxing a part of an anodeoff-gas of the solid oxide fuel cell to a water-containing ethanol vaporline from the mash column to the reforming device at a reflux ratio((flow rate of reflux gas)/(flow rate of (anode off-gas)−(reflux gas)))of from 1 to 2, so as to control an ethanol concentration of thewater-containing ethanol vapor to a range of from 25 to 35% by weightwith water contained in the anode off-gas of the solid oxide fuel cell.2. The energy saving method in a combined system of a bioethanolproducing device and a solid oxide fuel cell according to claim 1,wherein the ethanol concentration of the water-containing ethanol vapordistilled off from the mash column is controlled to a range of from 35to 60% by weight.
 3. The energy saving method in a combined system of abioethanol producing device and a solid oxide fuel cell according toclaim 1, wherein the ethanol concentration of the water-containingethanol vapor distilled off from the mash column is controlled to arange of from 55 to 60% by weight.
 4. The energy saving method in acombined system of a bioethanol producing device and a solid oxide fuelcell according to claim 1, wherein the balance of the anode off-gas ofthe solid oxide fuel cell and a cathode off-gas thereof are supplied toa catalytic combustor for the reforming device and a catalytic combustorfor a reboiler of the mash column, so as to combust a combustiblecomponent in the anode off-gas with oxygen in the cathode off-gas, andheat generated in the catalytic combustor for the reforming device isused for heating the reforming device, whereas heat generated in thecatalytic combustor for the reboiler is used for heating a bottom liquidof the mash column.
 5. The energy saving method in a combined system ofa bioethanol producing device and a solid oxide fuel cell according toclaim 2, wherein the balance of the anode off-gas of the solid oxidefuel cell and a cathode off-gas thereof are supplied to a catalyticcombustor for the reforming device and a catalytic combustor for areboiler of the mash column, so as to combust a combustible component inthe anode off-gas with oxygen in the cathode off-gas, and heat generatedin the catalytic combustor for the reforming device is used for heatingthe reforming device, whereas heat generated in the catalytic combustorfor the reboiler is used for heating a bottom liquid of the mash column.6. The energy saving method in a combined system of a bioethanolproducing device and a solid oxide fuel cell according to claim 3,wherein the balance of the anode off-gas of the solid oxide fuel celland a cathode off-gas thereof are supplied to a catalytic combustor forthe reforming device and a catalytic combustor for a reboiler of themash column, so as to combust a combustible component in the anodeoff-gas with oxygen in the cathode off-gas, and heat generated in thecatalytic combustor for the reforming device is used for heating thereforming device, whereas heat generated in the catalytic combustor forthe reboiler is used for heating a bottom liquid of the mash column.